1. Field of the Invention
The present invention relates to a method of annealing a compound semiconductor substrate. More specifically, it relates to a method of annealing a substrate after ion implantation in the process of fabricating a compound semiconductor integrated circuit.
2. Description of the Prior Art
Electronic devices have rapidly matured in recent years with semiconductor devices being as the forerunners. Except for an amorphous semiconductor device which is the most recent of practical semiconductor devices, electronic and optical devices generally have main operational regions, i.e., active regions formed by semiconductor single crystals. A plurality of such active regions are provided in a semiconductor substrate single crystal as regions that differ in electrical and/or optical properties from each other.
In the general method of fabricating an integrated circuit through the use of compound semiconductors, either an epitaxial growth process, a thermal diffusion process, an ion implantation process or the like is employed for forming an active layer, a resistance layer, a contact layer, etc. Of these processes, the ion implantation process is typically employed nowadays since the same is capable of correctly controlling the dose and injected depth of impurities, is operable at room temperature, excellent in uniformity and reproducibility of ion implantation, and short in process time.
The ion implantation process is performed by ionizing target impurity atoms and accelerating the same at an energy of 10 to several hundred KeV to implant the same in a semiconductor substrate, thereby doping the impurities into the semiconductor substrate. However, since the impurity ions are accelerated at such high energy as hereinabove described to be implanted in the substrate crystal, the ion implantation process has various problems to be solved in application to a compound semiconductor single crystal. For example, device characteristics are significantly affected by the type of implanted impurity atoms, deviation in stoichiometry of the crystal after ion implantation, annealing conditions including the type and film quality of a protective film for preventing vaporization of high vapor pressure component atoms, crystal quality of ion-implanted regions and the like.
With an increase in demand for devices operable at very high speed and at high frequency such as a GaAs FET (field-effect transistor) and a GaAs IC (integrated circuit), on the other hand, the ion implantation process has been positively employed as the fabrication technique for such devices. Thus, it is of urgent necessity to solve the aforementioned problems of the ion implantation process.
One of the problems of the ion implantation process is that, ion-implanted regions are degraded in crystallinity since the impurities are accelerated at high energy to be implanted in the substrate crystal, while regions entering amorphous states are increased in case of implanting the impurities in high concentration, whereby heat treatment is required to recover the degraded crystallinity and to substitute impurity atoms into lattice sites to obtain an electrically activated layer. In a general compound semiconductor substrate, however, one kind of component atoms have such high vapor pressure that the same are vaporized from the substrate during the heat treatment, whereby the surface of the substrate is remarkably disturbed in crystallinity (or in composition). As a result, implanted impurity ions cannot be constantly substituted into the substrate crystal to cause variations in electrical properties of devices, leading to a reduced production yield.
In order to solve the aforementioned problems, the following methods are generally employed:
(i) Method of applying a gas pressure: annealing ion-implanted layers in a gaseous atmosphere containing high vapor pressure component atoms of a compound semiconductor, and employing arsine (AsH.sub.3) to provide an As pressure atmosphere for a GaAs substrate, for example.
(ii) Method of facing surfaces: bringing an ion-implanted surface of a compound semiconductor substrate to be annealed oppositely into contact with the crystal or crystal powder of compound semiconductor containing high vapor pressure component atoms to anneal the same, thereby preventing vaporization of the high vapor pressure component atoms.
(iii) Protective film method: forming a protective film of silicon nitride, silicon oxide, silicon oxynitride or the like on the ion-implanted surface of a compound semiconductor substrate to be annealed through a CVD process, a sputtering process, a plasma CVD process or the like, to anneal the ion-implanted layers while preventing vaporization of high vapor pressure atoms from the substrate by the protective film.
(iv) Heat treatment through combination of the methods (i) and (iii) or the methods (ii) and (iii).
Method (i) employs an extremely toxic gas such as AsH.sub.3 as the atmospheric gas, which is to be diluted by hydrogen or an inactive gas in consideration of safety and workability. Thus, a sufficient vapor pressure cannot be applied and hence vaporization of the high vapor pressure atoms cannot be sufficiently prevented.
Such a problem similarly takes place in method (ii) of facing surfaces, and hence it is difficult to prevent vaporization of the high vapor pressure component atoms from the substrate to be annealed only by the vapor pressure from the crystal or the crystal powder.
Therefore, methods (iii) or (iv) are most generally employed. However, methods (iii) and (iv) both have disadvantages such as that the protective film is inferior in quality and cannot sufficiently prevent vaporization of the high vapor pressure component atoms and that remaining low vapor pressure component atoms are diffused into the protective film.
As hereinabove described, the ion implantation process is indispensable as the process technique for fabricating various devices from compound semiconductor single crystal substrates, but the same has not yet been completely established, with various problems remaining to be solved. Although the aforementioned various methods have been proposed and carried out particularly in heat treatment for recovering crystallinity of regions degraded after ion implantation and for electrically activating implanted ions, the relatively effective methods (iii) and (iv) still have many problems to be solved.
Thus, development of a novel annealing method capable of solving such problems is extremely useful not only for finally obtaining a semiconductor device of high efficiency and high reliability, but for improving the production yield thereof to save costs.